The present invention relates to an optical system to be employed in an optical disc drive to read/write data from/to an optical disc.
Recently, technology in the field of magneto-optical disc drives has been greatly improved such that a data recording density on a magneto-optical disc has reached in excess of 10 Gbits/inch.sup.2.
In an example of such an optical disc drive, an objective optical system is mounted on an arm which is movable in a transverse direction of tracks formed on an optical disc for rough tracking. Firstly, the rough tracking is performed to locate the optical head in the vicinity of the track.
Then, an incident angle of a beam incident on the objective optical system is controlled to locate a beam spot formed by the objective optical system for fine tracking, with use of a galvano mirror or the like. During the fine tracking operation, the beam spot is accurately located on one of the tracks whose pitch is, for example, 0.34 .mu.m.
In the optical disc drive as described above, however, if a rotation amount of the galvano mirror exceeds a predetermined range, optical performance thereof is remarkably lowered. Thus, it is necessary to detect the rotation amount of the galvano mirror, and the rotation amount should be controlled to be maintained within the predetermined range.